Just a quick heads up in case you are not aware of this but any recent Paypal accounts created since around November 2009 no longer send out subscr_eot IPN messages when a recurring payment is cancelled. This means that it's not possible to automatically demote a member upon their subscription running out.
i.e. say a member pays for a months sub then cancels after 3 days, you will only receive a subscr_cancel IPN on the day they cancel and no subscr_eot at the end of the month when their subscription should run out. This may effect members not getting the actual time on a site they paid for if they cancel during a cycle.
Just wanted to let you know incase you were testing your new plugin in an old sandbox account as this won't reflect the new live environment.
You'll probably be aware of this anyway but I have ntoiced other subscription plugins struggling with this so wanted to make you aware so your plugin can be the best!